Is an upgrade from MacOS Mojave 10.14.6 to OS Monterey possible

Try installing in Safe Mode..

macOS High Sierra - Technical Specifications

General Requirements

• OS X 10.8 or later
• 2GB of memory
• 14.3GB of available storage to perform upgrade*
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:

• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)

macOS High Sierra - Technical Specifications

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

BDAqua wrote:

Try installing in Safe Mode..

macOS High Sierra - Technical Specifications

FWIW, I suggested Safe Mode about six hours before your post. I would also mention the user has a Mid 2015 MacBook Pro and wants to upgrade from Mojave to Monterey. 😎

Not sure what the issue is but the user reported Safe Mode didn't help.

Yes, it's possible. Be aware older 32bit apps that would run in Mojave will not run in Monterey and will need updated or replaced. You can check which you might have with Go64.

Delete any copies of Install macOS Monterey from the Applications folder, then try again while in Safe Mode.

How to use safe mode on your Mac - Apple Support

Missed that I guess.

I think the OP should try froma USB stick..

Create a bootable installer for macOS - Apple Support

I think you should try from a USB stick..

Create a bootable installer for macOS - Apple Support

OK, let us know if you need further help.